The measure NSPACE is used to define the complexity class whose solutions can be determined by a non-deterministic Turing machine.
The complexity class NSPACE(f(n)) is the set of decision problems that can be solved by a non-deterministic Turing machine, M, using space O(f(n)), where n is the length of the input.
These include: The Immerman–Szelepcsényi theorem states that NSPACE(s(n)) is closed under complement for every function s(n) ≥ log n. A further generalization is ASPACE, defined with alternating Turing machines.
The reason is that DSPACE describes the space complexity used by deterministic Turing machines, which can represent actual computers.
On the other hand, NSPACE describes the space complexity of non-deterministic Turing machines, which are not useful when trying to represent actual computers.